Stabilizing Emulsified Acids for Carbonate Acidizing

ABSTRACT

Emulsified acids have been used to increase production rates of oil and gas in carbonate reservoirs through acid fracturing and matrix acidizing operations. An emulsifier is used to emulsify the aqueous acid with an oil, usually diesel. Very small particles, such as colloidal clay particles and/or nanoparticles increase the stability of the emulsified acids over an elevated temperature range.

TECHNICAL FIELD

The present invention relates to acidizing treatment fluids used duringhydrocarbon recovery operations, and more particularly relates, in oneembodiment, to emulsified acid compositions and methods of using them totreat a subterranean formation, where the emulsified acid compositionhas improved thermal stability.

BACKGROUND

Hydrocarbons sometimes exist in a formation but cannot flow readily intothe well because the formation has very low permeability. Acidizingwells is a conventional process for increasing or restoring thepermeability of subterranean formations so as to facilitate the flow ofoil and gas from the formation into the well. This process involvestreating the formation with an acid to dissolve fines and carbonatescale that are plugging or clogging the pores, thereby opening the poresand other flow channels and increasing the permeability of theformation. Continued pumping forces the acid into the formation, whereit etches channels or wormholes. These channels provide ways for theformation hydrocarbons to enter the well bore.

Conventional acidizing fluids, such as hydrochloric acid or a mixture ofhydrofluoric and hydrochloric acids, have high acid strength and quickreaction with fines and scale nearest the well bore, and have a tendencyto corrode tubing, casing and downhole equipment, such as gravel packscreens and downhole pumps, especially at elevated temperatures. Inaddition, above 200° F. (92° C.), HCl is not recommended in some casesbecause of its destructive effect on the rock matrix. Due to the type ofmetallurgy, long acid contact times and high acid sensitivity of theformations, removal of the scale with hydrochloric acid and hydrochloricacid mixtures has been largely unsuccessful. However, there are otheracid fluid systems to dissolve carbonate and/or scale and remove thesource of the fines through acidizing the surrounding formation whilenot damaging the downhole hole equipment, particularly for hightemperature wells. These acid systems include, but are not necessarilylimited to mixtures of tricarboxylic acids, aminocarboxylic acids,dicarboxylic acids including, but not necessarily limited to, oxalicacid (ethanedioic acid), malonic acid (propanedioic acid), succinic acid(butanedioic acid), glutaric acid (pentanedioic acid), adipic acid(hexanedioic acid), pimelic acid (heptanedioic acid), and mixturesthereof. Further details about these acids may be had with reference toU.S. Pat. No. 6,805,198 assigned to Baker Hughes Incorporated, and it isincorporated herein by reference.

Other acids used to treat subterranean formations include, but are notnecessarily limited to sulfuric acid, phosphoric acid, hydrofluoricacid, hydrobromic acid, fluoroboric acid, formic acid, acetic acid,glycolic acid, and mixtures of these and the forementioned acids.

It is also known to use emulsified fluids containing such acids to treatsubterranean formations. Emulsified acids give reduced or retarded acidreaction rates because of the oil-external outer phase. Some of theseacids mentioned above have reduced acid reaction rates with carbonatereservoirs that in turn allows deeper and narrower wormholes to formthat may include secondary microfractures. The longer wormholes and/orsecondary microfractures are needed in order to improve hydrocarbonproduction rates. However, a difficulty with some emulsified acids isthat they are not thermally stable, that is the viscosity changesmarkedly over time at elevated temperature.

It would be desirable if a composition and method could be devised toovercome some of the problems in the conventional acidizing methods andfluids, particularly with respect to thermal stability.

SUMMARY

There is thus provided in one non-limiting embodiment a method foracidizing a subterranean carbonate-containing formation, where themethod includes injecting a stabilized emulsified acid into thesubterranean carbonate-containing formation. The stabilized emulsifiedacid includes at least one oil, an aqueous acid solution containing atleast one acid, at least one emulsifier, and acid insolublenanoparticles, having an average particle size of 1000 nanometers orless. The amount of nanoparticles is effective to improve the thermalstability of the stabilized emulsified acid as contrasted with anidentical emulsified acid absent the nanoparticles. The method furtherincludes dissolving at least a portion of the carbonate in the formationwith the acid.

Further there is also provided in another non-restrictive form astabilized emulsified acid per se that includes at least one oil, anaqueous acid solution having at least one acid, at least one emulsifier,and acid insoluble nanoparticles having an average particle size of 1000nanometers or less. Again, the amount of nanoparticles is that effectiveto improve the thermal stability of the stabilized emulsified acid ascontrasted with an identical emulsified acid absent the nanoparticles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of the viscosities of two emulsified acids at 200° F.(93° C.) and 100 1/s as a function of time, where the black curve(diamonds) shows the results from an emulsified acid with nano-sizedclay particles and the gray curve (squares) shows the results from anidentical emulsified acid but without nano-sized clay particles; and

FIG. 2 is a graph of the viscosities of three emulsified acids at 150°F. (66° C.) and 100 1/s as a function of time, where the black curve(diamonds) shows the shear rate, the black curve (triangles) shows theviscosity of an emulsified acid without clay nanoparticles including 3%AQUET® 942 acid emulsifier, the gray curve (squares) shows the viscosityof an emulsified acid with 0.75% clay nano-particles including 3% AQUET®942 acid emulsifier, and the gray curve (Xs) shows the viscosity of anemulsified acid with 0.75% clay nanoparticles but not including 3%AQUET® 942 acid emulsifier.

DETAILED DESCRIPTION

Emulsified acids have been used to increase production rates of oil andgas in carbonate-containing reservoirs through acid fracturing andmatrix acidizing operations. The acids emulsified with oil, usuallydiesel, can reduce corrosion of pumping equipment and downhole tubulars,and decrease the reaction rates with formation carbonates at highertemperatures, which provides active acid to penetrate deeply into theformation. As noted, emulsified acids give reduced or retarded acidreaction rates because of the oil-external outer phase. Thestabilization of acid and oil emulsions is usually an issue for fieldapplications. By “stable” it is meant that the viscosity of theemulsified acid is generally maintained, and does not markedly increaseor decrease, even over elevated temperatures and the expected pumpingshear. A temperature range of interest includes, but is not necessarilylimited to, about 100° F. to about 300° F. (about 38° C. to about 149°C.), or alternatively up to about 200° F. (93° C.).

It has been discovered that very small particles, such as colloidalparticles and/or nanoparticles increase the temperature stability ofemulsified acids. In one non-limiting embodiment, the nanoparticles havean average particle size of about 1000 nanometers (nm) or less,alternatively about 800 nm or less, in another non-restrictive versionabout 600 nm or less or in another form about 400 nm or less. In onenon-limiting embodiment, colloidal particles are defined as having anaverage particle size of between about 5 independently to about 200 nm.Alternatively, a lower size threshold is about 10 nm. The effectiveamounts of these nanoparticles in the emulsified acid may range fromabout 0.1 independently to about 5 wt %, alternatively from about 0.5independently to about 2 wt %. The term “independently” as used hereinwith respect to ranges means that any lower threshold may be combinedwith any upper threshold.

The very small particles should be essentially acid insoluble. By“essentially acid insoluble” is meant less than 1 wt % soluble. Suitableacid insoluble particles are clays, a specific non-limiting example ofwhich is high purity montmorillonite. Other suitable materials mayinclude, but are not limited to, nano-sized quartz, tungsten oxide,graphite, graphene, graphene oxide, nano-carbon tubes, andnano-diamonds. In one non-limiting embodiment, the nanoparticles mayhave high surface areas compared to their small sizes, but also haverelatively high surface charges that permit them to associate or connectother substances together. In one non-limiting embodiment, theseassociations or connections may help stabilize the emulsified acids andmay be due to electrical attractions and other intermolecular forces oreffects, but this is a non-limiting theory.

These tiny particles stay with the emulsifier that is positioned betweenthe aqueous acid and the oil to increase the strength of the interfaceto maintain the stability of the acid emulsion at high temperatures.Thus, the water-in-oil (w/o) emulsion is not as easy to break. As willbe discussed further below, laboratory tests with colloidal clayparticles show that the stability of emulsified acids such as 20 vol %hydrochloric (HCl) acid at temperatures of 200° F. (92° C.) and 10 vol %HTOTM acid is significantly increased at 0.75 wt % of the colloidalparticles. This particle-stabilizing emulsion method may also used toimprove fluid loss control, frac-pack and gravel packing fluid systems.HTOTM acid, available from Baker Hughes Incorporated, includesdicarboxylic acids useful in acidizing subterranean formations toimprove their permeability, particularly at elevated temperature, e.g.between about 200° F. (92° C.) and about 400° F. (204° C.), as describedin U.S. Pat. No. 6,805,198 to Tianping Huang, et al., incorporatedherein in its entirety by reference. Particularly suitable dicarboxylicacids described in this patent include, but are not necessarily limitedto, succinic acid, glutaric acid, adipic acid, and mixtures thereof.

Some emulsified acid fluid systems have a primary emulsifier and oftenan optional secondary emulsifier to achieve a thermally stable emulsionwith an oil external phase and an acid internal phase. Typically, theacid internal phase may also comprise water, that is, the acid is inaqueous solution. The oil external phase may suitably be xylene, diesel,toluene, kerosene, other aromatics, refined hydrocarbons and mixturesthereof. Other suitable refined hydrocarbons and other aromatichydrocarbons may include, but are not necessarily limited to, organicfluids such as aliphatic, alicyclic and aromatic hydrocarbons, esters,derivatives of these compounds, as well as mixtures thereof. Specificexamples of suitable aliphatic hydrocarbons include, but are notnecessarily limited to, alkanes such as n-octane, n-decane, n-tridecane,and higher carbon number alkanes. Other suitable aliphatic hydrocarbonsinclude alkenes and alkadienes. Alicyclic compounds include cyclohexane,and the like. Specific examples of suitable aromatics include, but arenot necessarily limited to, benzene, toluene, xylene, ethylbenzene andother alkyl benzenes, naphthalene, and the like. Other examples includenonylphenols, ethoxylated nonylphenol, and the like. Particular examplesof commercial aromatic products include, but are not necessarily limitedto, FRACSOL, FRACSOL-S, XYSOL from Trysol of Calgary, Canada or GeoStimof the United States.

Other specific examples of suitable organic fluids for the oil phaseinclude, but are not necessarily limited to, at least one of diesel,gasoline, kerosene, reformate, naphthalene, xylene, toluene, mineraloil, light mineral oil, condensate, crude oil, lubricating oils, ormixtures thereof (such as diesel mixed with condensate to lower APIgravity, etc.).

Also suitable are synthetic oils (including, but not necessarily limitedto, synthetic hydrocarbon-based oils, ester-type oils, and the like.).Also suitable are more environmentally compatible (e.g., biodegradable)natural or synthetic organic fluids such as ExxonMobil's ESCAID 90 orESCAID 110, or refined kerosene (such as LOTOX available from Exxon),ALPHA OLEFIN (from Baker Performance Chemical), HYDROSOLVE P150 orHYDROSOLVE B100 (from Shrieve Chemical Products), ISOPAR L or ISOPAR M(from Nalco-Exxon Chemical Company), and the like. Natural organic basedfluids such as animal oils and vegetable oils may also be suitableincluding, but not necessarily limited to, linseed oil, palm oil, cottonseed oil, rapeseed oil, soybean oil, olive oil, canola oil, sunfloweroil, peanut oil, and the like. Other aromatic and refined hydrocarbonsare described in U.S. Pat. No. 6,849,581, incorporated by referenceherein in its entirety.

Suitable acids for use herein include, but are not necessarily limitedto, hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid,hydrofluoric acid, hydrobromic acid, boric acid, fluoroboric acid,formic acid, acetic acid, glycolic acid, dicarboxylic acids (including,but not necessarily limited to, oxalic acid, malonic acid, succinicacid, glutaric acid, adipic acid, pimelic acid and the like),tricarboxylic acids, aminocarboxylic acids, sulfonic acids, chloraceticacid, hydroxyacetic acid and mixtures thereof. In one non-limitingembodiment the acid concentration in the aqueous phase is usuallygreater than about 10%. An acid concentration of less than about 5% isnot used in other non-limiting versions.

In another non-restrictive version herein, the pH of emulsified acidsmay range from about 0.01 independently to about 3.0, alternatively fromabout 0.1 independently to about 1.5. A pH range of between 5 and 7 maybe considered too high in most cases.

Similar to other emulsified acid systems, the present fluids havereduced acid reaction rates with carbonate reservoirs that in turnpermit deep and narrow wormholes to form that may include secondarymicrofractures. The longer wormholes and secondary microfracturesinvolve good clean-up in order to improve hydrocarbon production rates.

Suitable emulsifiers include, but are not necessarily limited to, lowHLB surfactants or oil soluble surfactants. In one non-limitingembodiment the HLB is about 12 or below; alternatively, the HLB is about10 or below; and in another non-limiting embodiment is about 8 or below.More specific suitable emulsifiers include, but are not necessarilylimited to, polysorbates, alkyl sulfosuccinates, alkyl phenols,ethoxylated alkyl phenols, alkyl benzene sulfonates, fatty acids,ethoxylated fatty acids, propoxylated fatty acids, fatty acid salts,tall oils, castor oils, triglycerides, ethoxylated triglycerides, alkylglucosides, and mixtures and derivatized fatty acids such as thosedisclosed in U.S. Pat. No. 6,849,581, incorporated by reference hereinin its entirety. Suitable polysorbates include, but are not necessarilylimited to, sorbitan monolaurate, sorbitan monopalmitate, sorbitanmonostearate, sorbitan monooleate, sorbitan monodecanoate, sorbitanmonooctadecanoate, sorbitan trioleate and the like and ethoxylatedderivatives thereof. For instance, these emulsifiers may have up to 20ethoxy groups thereon. Suitable polysorbates include, but are notnecessarily limited to, SPAN® 40, SPAN 40, SPAN 60 and SPAN 80polysorbates available from Croda International PLC. Other suitableemulsifiers include stearyl alcohol, lecithin, fatty acid amines,ethoxylated fatty acid amines and mixtures thereof. In one non-limitingembodiment, more than one emulsifier is used. Matching which emulsifierswork well together to give the desired results may largely be a matterof trial and error.

In one non-limiting embodiment, the stabilized emulsified acids do notcontain any viscoelastic surfactant, alternatively less than 0.1 vol %,or in another non-limiting embodiment less than 0. 5 vol % viscoelasticsurfactant.

In one non-limiting embodiment, the concentration of aqueous acid in theemulsified fluid is greater than 10 wt %. The amount of aqueous acid maybe between about 50 independently to about 85 vol %, and alternativelybetween about 60 independently to about 75 vol %. The amount ofhydrocarbon, e.g. diesel in the emulsified acid may range from about 13independently to about 45 vol %, alternatively from about 20independently to about 35 vol %. The amount of emulsifier may range fromabout 1 independently to about 5 vol %, alternatively from about 2independently to about 4 vol %, all based on the total of the emulsifiedacid.

The viscosity of the emulsified acid should be stable during pumpingthrough the tubing and during flow into the formation, which is a timeframe of about 3 to 4 hours, in one non-limiting embodiment. In onenon-restrictive version, the viscosity ranges from about 30independently to about 200 cP; alternatively from about 50 independentlyto about 150 cP.

It will be appreciated that it is not necessary for the viscosity of theemulsified acid to be exactly stable at only one viscosity for thecompositions and methods described herein to be considered successful.However, the viscosity of the emulsified acid is expected to vary over amuch narrower range over time at elevated temperature to be successful,as compared to an otherwise identical emulsified acid having nocolloidal or nanosized particles. In one non-limiting embodiment, theviscosity may vary ±15 cP, alternatively ±10 cp and in anothernonrestrictive version ±5 cP.

In another non-restrictive version, the emulsified acids do not include,or have an absence of a gas, for instance an absence of a gas, such asto adjust, particularly decrease, viscosity; in particular an absence ofcarbon dioxide, nitrogen, air, flue gas, and combinations thereof.

The emulsified acids may effectively generate wormholes to stimulateproduction in subterranean carbonate formations and dissolve carbonateand/or scale, and these emulsified acid systems may effectively removefines to recover production in sandstone formations at elevatedtemperatures. In one nonlimiting aspect, the emulsified acids delay orinhibit the quick reaction of the acid with wellbore and formationcarbonate rocks, thus permitting the fluid to have deeper penetrationand to permit the acid to be effective for a longer distance.

It will be appreciated that it is difficult to specify with precisionthe amount of emulsified acid that must be used to effectively acidize aparticular subterranean formation. A number of complex, interrelatedfactors must be taken into account that would affect such a proportion,including but not necessarily limited to, the temperature of theformation, the pressure of the formation, the particular fines andscales present in the formation (e.g. calcium carbonate, silicates, andthe like), the particular dicarboxylic acid(s) used, the particular kindof rock in the formation, the expected contact time of the acidcomposition with the formation, etc.

For stimulation treatments, contact times are determined from themaximum pumping rate that does not cause the downhole pressure to exceedthe fracturing pressure. This type of treatment is called a “matrix”acid job.

For scale/fines removal procedures, contact times are based onlaboratory tests, but usually range from about 0.5 hour to about 2 hourwith the most common time being about 0.5 hour.

The invention will be further illustrated with respect to certainexperiments, but these examples are not intended to limit the invention,but only to further describe it in certain specific, non-limitingembodiments.

EXAMPLE 1

Shown in FIG. 1 is a graph of the viscosities of two emulsified acids at200° F. (93° C.) and 100 1/s as a function of time. The base fluid is a20% HCl acid having 1% corrosion inhibitor (CI-300 corrosion inhibitor,a product of Baker Hughes Incorporated), 1% iron control agent(ethylenediaminetetraacetic acid, EDTA) and 2.5% AQUET® 942 emulsifier(also available from Baker Hughes Incorporated) and 30% diesel oil. Thegray curve (squares) shows the results of the emulsified acid withoutnano-sized clay particles, and it may be seen that the viscosity changesmarkedly over time at this temperature. In contrast, when 0.75% MO clayproduct (a mineral colloid where the clay particles have an averageparticle size of less than 1000 nm, available from Southern ClayProducts, inc.) was added, the data in the black curve (diamonds)resulted, showing that the viscosity was much more stable and consistentover time at this temperature; between about 90-100 cP.

EXAMPLE 2

FIG. 2 presents the viscosities of emulsified acids at 150° F. (66° C.)and 100 1/s as a function of time. The black curve (diamonds) shows theshear rate. An emulsified acid made with 30% diesel, 10% aqueous HTOacid and 3% QT (AQUET® 942 emulsifier), but no nanoparticles gave theblack curve with triangles at the bottom of the graph, and wasessentially non-viscous. A formulation with 10% HTO, 30% diesel and0.75% MO colloidal clay, but no emulsifier, gave the gray curve (Xs),but this fluid also had very low viscosity. However, the emulsified acidhaving 30% diesel, 10% aqueous HTO acid, 0.75% MO colloidal clay and 3%QT (AQUET® 942 emulsifier) gave the gray curve (squares) demonstrating astable viscosity at around 150 cP at this temperature, furtherdemonstrating the effectiveness of the compositions described herein.

In the foregoing specification, the invention has been described withreference to specific embodiments thereof, and has been demonstrated aseffective in providing emulsified acid systems that have improvedstability. However, it will be evident that various modifications andchanges can be made thereto without departing from the broader spirit orscope of the invention as set forth in the appended claims. Accordingly,the specification is to be regarded in an illustrative rather than arestrictive sense. For example, specific combinations of acids,hydrocarbon phases, emulsifiers, colloidal and nanosized particles andother components falling within the claimed parameters, but notspecifically identified or tried in a particular composition or underspecific conditions, are anticipated to be within the scope of thisinvention.

The present invention may suitably comprise, consist or consistessentially of the elements disclosed and may be practiced in theabsence of an element not disclosed. For example the stabilizedemulsified acid may consist essentially of or consist of at least oneoil, an aqueous acid solution having at least one acid, at least oneemulsifier, and an amount of acid insoluble nanoparticles as defined bythe claims.

The words “comprising” and “comprises” as used throughout the claims isto interpreted “including but not limited to”.

1. A method for acidizing a subterranean carbonate-containing formation,the method comprising: injecting a stabilized emulsified acid into thesubterranean carbonate-containing formation, where the stabilizedemulsified acid comprises: at least one oil; an aqueous acid solutioncomprising at least one acid; at least one emulsifier; and acidinsoluble nanoparticles, having an average particle size of about 1000nanometers or less, where the acid insoluble nanoparticles are selectedfrom the group consisting of clays, quartz, tungsten oxide, graphite,graphene, graphene oxide, nano-carbon tubes, and nano-diamonds andcombinations thereof; and dissolving at least a portion of the carbonatein the formation with the acid.
 2. The method of claim 1 where theamount of acid insoluble nanoparticles is effective to improve thethermal stability of the stabilized emulsified acid as contrasted withan identical emulsified acid absent the nanoparticles.
 3. The method ofclaim 1 where the at least one oil is selected from the group consistingof xylene, diesel, toluene, kerosene, other aromatics, refinedhydrocarbons and mixtures thereof.
 4. The method of claim 1 where the atleast one acid is selected from the group consisting of hydrochloricacid, sulfuric acid, nitric acid, phosphoric acid, hydrofluoric acid,hydrobromic acid, boric acid, fluoroboric acid, formic acid, aceticacid, glycolic acid, dicarboxylic acid, tricarboxylic acids,aminocarboxylic acids, sulfonic acids, chloracetic acid, hydroxyaceticacid and mixtures thereof.
 5. The method of claim 1 where the at leastone emulsifier is selected from the group consisting of polysorbates,alkyl sulfosuccinates, alkyl phenols, ethoxylated alkyl phenols, alkylbenzene sulfonates, ethoxylated fatty acid amines, fatty acid amines,stearyl alcohol, lecithin, fatty acids, ethoxylated fatty acids,propoxylated fatty acids, fatty acid salts, tall oils, castor oils,triglycerides, ethoxylated triglycerides, alkyl glucosides, and mixturesand derivatives thereof.
 6. (canceled)
 7. The method of claim 1, wherethe stabilized emulsified acid comprises: from about 13 to about 45 vol% of the at least one oil; from about 50 to about 85 vol % of the atleast one aqueous acid solution; from about 1 to about 5 vol % of the atleast one emulsifier; and from about 0.1 to about 5 wt % of acidinsoluble nanoparticles.
 8. The method of claim 1 where the acidconcentration in the aqueous acid solution is 10 vol % or greater, basedon the aqueous acid solution.
 9. The method of claim 1 where theemulsified acid has a viscosity in the range of from about 30 to about200 cP.
 10. A method for acidizing a subterranean carbonate-containingformation, the method comprising: injecting a stabilized emulsified acidinto the subterranean carbonate-containing formation, where thestabilized emulsified acid comprises: from about 13 to about 45 vol % ofat least one oil selected from the group consisting of xylene, diesel,toluene, kerosene, other aromatics, refined hydrocarbons and mixturesthereof; from about 50 to about 85 vol % of an aqueous acid solutioncomprising at least one acid is selected from the group consisting ofhydrochloric acid, sulfuric acid, nitric acid, phosphoric acid,hydrofluoric acid, hydrobromic acid, boric acid, fluoroboric acid,formic acid, acetic acid, glycolic acid, dicarboxylic acid,tricarboxylic acids, aminocarboxylic acids, sulfonic acids, chloraceticacid, hydroxyacetic acid and mixtures thereof; from about 1 to about 5vol % of at least one emulsifier selected from the group consisting ofpolysorbates, alkyl sulfosuccinates, alkyl phenols, ethoxylated alkylphenols, alkyl benzene sulfonates, ethoxylated fatty acid amines, fattyacid amines, stearyl alcohol, lecithin, fatty acids, ethoxylated fattyacids, propoxylated fatty acids, fatty acid salts, tall oils, castoroils, triglycerides, ethoxylated triglycerides, alkyl glucosides, andmixtures and derivatives thereof; and from about 0.1 to about 5 wt % ofacid insoluble nanoparticles, having an average particle size of about1000 nanometers or less, where the nanoparticles are selected from thegroup consisting of clays, quartz, tungsten oxide, graphite, graphene,graphene oxide, nano-carbon tubes, and nano-diamonds and combinationsthereof; and dissolving at least a portion of the carbonate in theformation with the acid.
 11. The method of claim 10 where the acidconcentration in the aqueous acid solution is 10 vol % or greater, basedon the aqueous acid solution.
 12. The method of claim 10 where theemulsified acid has a viscosity in the range of from about 30 to about200 cP. 13-21. (canceled)
 22. A method for acidizing a subterraneancarbonate-containing formation, the method comprising: injecting astabilized emulsified acid into the subterranean carbonate-containingformation, where the stabilized emulsified acid comprises: from about 13to about 45 vol % of at least one oil; from about 50 to about 85 vol %of an aqueous acid solution comprising at least one acid; from about 1to about 5 vol % of at least one emulsifier; and from about 0.1 to about5 wt % of acid insoluble nanoparticles, having an average particle sizeof about 1000 nanometers or less, where the acid insoluble nanoparticlesare selected from the group consisting of clays, quartz, tungsten oxide,graphite, graphene, graphene oxide, nano-carbon tubes, and nano-diamondsand combinations thereof; and dissolving at least a portion of thecarbonate in the formation with the acid; where the amount of acidinsoluble nanoparticles is effective to improve the thermal stability ofthe stabilized emulsified acid as contrasted with an identicalemulsified acid absent the nanoparticles.
 23. The method of claim 22where the at least one oil is selected from the group consisting ofxylene, diesel, toluene, kerosene, other aromatics, refined hydrocarbonsand mixtures thereof.
 24. The method of claim 22 where the at least oneacid is selected from the group consisting of hydrochloric acid,sulfuric acid, nitric acid, phosphoric acid, hydrofluoric acid,hydrobromic acid, boric acid, fluoroboric acid, formic acid, aceticacid, glycolic acid, dicarboxylic acid, tricarboxylic acids,aminocarboxylic acids, sulfonic acids, chloracetic acid, hydroxyaceticacid and mixtures thereof.
 25. The method of claim 22 where the at leastone emulsifier is selected from the group consisting of polysorbates,alkyl sulfosuccinates, alkyl phenols, ethoxylated alkyl phenols, alkylbenzene sulfonates, ethoxylated fatty acid amines, fatty acid amines,stearyl alcohol, lecithin, fatty acids, ethoxylated fatty acids,propoxylated fatty acids, fatty acid salts, tall oils, castor oils,triglycerides, ethoxylated triglycerides, alkyl glucosides, and mixturesand derivatives thereof.
 26. The method of claim 11 where the acidconcentration in the aqueous acid solution is 10 vol % or greater, basedon the aqueous acid solution.
 27. The method of claim 11 where theemulsified acid has a viscosity in the range of from about 30 to about200 cP.